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Genetics Continued

Genetics Continued. Test Cross Codominance Incomplete Dominance Sex Determination Sex-Linked Dihybrid Cross. Test Cross. Remember: The dominant phenotype has 2 possible genotypes. Example: TT OR Tt

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Genetics Continued

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  1. Genetics Continued Test Cross Codominance Incomplete Dominance Sex Determination Sex-Linked Dihybrid Cross

  2. Test Cross • Remember: The dominant phenotype has 2 possible genotypes. • Example: TT OR Tt • When trying to find out the genotype of a dominant phenotype ALWAYS cross it with the homozygous recessive. • Then look at the children because they will tell you if the parent is homozygous dominant or heterozygous.

  3. Example of a Test Cross Problem • In Sharks, sharp teeth are dominant (T) to dull teeth (t). If I found a sharp toothed shark, how can I find out its genotype? Prove it with Punnet Squares.

  4. Answer: Cross it with a homozygous recessive (tt), then look at the children. T T T t Tt t Tt Tt tt t t Tt tt t Tt Tt All Children Sharp Teeth 50% Sharp 50% Dull

  5. Codominance • Codominance means both phenotypes are dominant. • Example: Orange goldfish (OO) are codominant with white goldfish (WW). A heterozygous goldfish (OW) would be both Orange and White (spotted).

  6. Incomplete Dominance • Incomplete dominance means that neither phenotype is dominant over the other. • It is a blending of the phenotypes. • Example: Red octopus (RR) are incompletely dominant to Blue octopus (R’R’). A heterozygous (RR’) would be purple (Red + Blue = Purple)

  7. Sex Determination • All animals have 1 pair of chromosomes that determine their sex. • In MOST animals, females are XX and males are XY. • Therefore, when crossing a male (XY) with a female (XX), what is the chance of having a boy? A girl? Who passes the chromosome to determine sex, Mom or Dad?

  8. Answer: DAD MOM X X • 50% Chance for a boy • 50% Chance for a girl • Dad determines the sex (gives X or Y). Mom can only give an X. X XX XX XY XY Y

  9. Sex Linked • There are some traits that are located on the sex chromosomes (X or Y), which are called sex linked. • These traits are often common in men because they only have one X chromosome, but women have two X chromosomes to protect them.

  10. Example Sex Linked Problem • Albinism (lack of skin color) is a recessive sex linked trait carried on the X chromosome (Xa). If a dolphin is not an albino they have a dominant X chromosome (XA). • A heterozygous female dolphin is crossed with a normal male dolphin. What is the chance the girl dolphins will be albino? What is the chance the boy dolphins will be albino?

  11. Answer: DAD MOM XA Xa XA XAXA XAXa XAY XaY Y • 0% chance that the girls will be an albino • 50% chance of the boys will be an albino

  12. Dihybrid Cross • A dihybrid cross is a 16 box Punnet Square • It is used to cross 2 different traits at the same time • Ex: Height and Color • Write the genotypes of the parents • Write the possible gametes of each parent Remember: 1st-1st, 1st-2nd, 2nd-1st, 2nd-2nd • Write the gametes outside the Punnet Square • Then fill in the boxes like a Punnet Square

  13. Remember: 1st-1st, 1st-2nd, 2nd-1st, 2nd-2nd • Practice: TtFf TtFf TF Tf tF tf

  14. TtFf x TtFfT=sharp, t=dull F=fancy, f=plain TTFF TTFf TtFF TtFf TTFf TTff TtFf Ttff TtFF TtFf ttFF ttFf TtFf Ttff ttFf ttff

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